1. Field of the Invention
The present invention generally relates to a scanning optical apparatus for scanning light beams, and particularly relates to a scanning optical apparatus employable in an electrophotographic-type image forming device.
2. Description of the Related Art
In recent years, the productivity of exposure units in an image forming apparatus such as printers and copiers has become increasingly important. The productivity of an exposure unit is determined by a number of lines to be exposed (number of scans) on an image carrier (a photosensitive body formed that is a photosensitive drum) per unit time. Conceivable methods to improve the productivity include increasing the rotation speed of a polygon motor, and increasing the number of faces of a polygonal mirror (rotating multifaceted mirror).
However, the rotation speed of the polygonal motors is currently approaching a limiting value. Further increasing the rotation speed would bring about increases in temperature, noise level and cost, and it is not therefore desirable to do so. Also, since increasing the number of faces of the polygonal mirror results in a reduction in a scanning angle for a single scan on the image carrier, optical path had to be lengthened to obtain the same exposure width. This leads to increases in the size and cost of the apparatus.
To deal with such problems, a multibeam method has been proposed (Japanese Patent Laid-Open No. 2006-116716) in which the number of beams used in a single scan is increased. Conventionally, laser diodes with an edge-emitting construction have been mainly used. To realize the multibeam method, however, laser diodes with a surface-emitting construction (hereinafter referred to as surface-emitting lasers) have been developed. The use of surface-emitting lasers is expected to improve the productivity of exposure units.
It is also desirable to improve resolution. To do so, it is necessary to reduce dot size. However, it is not possible to reduce the dot size only by using the surface emitting laser. One method for reducing the size of the dots makes use of multiple exposures.
FIG. 14 is a diagram showing an example of a single exposure performed using a conventional single scan. A negative charge is applied in advance to the photosensitive drum. When the negatively charged drum is irradiated with laser light, positive charges are generated in a CG layer (charge generation layer). The positive charges generated in the CG layer combine with the negative charges on the surface of the drum via a CT layer (charge transfer layer). This causes a drop in potential at the surface of the photosensitive drum. When the amount of laser light is large at this point, a greater number of positive charges are generated. These positive charges then repel one another, causing the charge to spread when passing through the CT layer. Consequently, the spots of the latent image are undesirably larger in diameter than the exposed spots.
FIG. 15 is a diagram showing an example of a multiple exposure. In multiple exposure, the exposure is implemented by performing a plurality of exposures. Hence, the amount of laser light used in each of the plurality of exposures is less than the amount in the example shown in FIG. 15. Since fewer positive charges are generated in each exposure, the amount of charge which spreads when passing through the CT layer is reduced. Hence, the latent image spot diameter produced by the multiple exposures is smaller than a latent image spot diameter produced by the single exposure.
FIG. 16 is a diagram showing a surface-emitting laser beam arrangement for performing the multiple exposures, according to the related art. The emitting surface of the surface-emitting laser beam lies in the X-Y plane. Eight light-emitting portions denoted A1 to D2 are arranged in the X-Y plane. Two light-emitting portions are provided in each of rows A through D. For example, the light-emitting portions A1 and A2 have the same Y-coordinate. The distances P between the rows are identical. The X-coordinate differs for each light-emitting portion.
Here, a main scanning direction on the photosensitive body (a longitudinal direction of the photosensitive body) is assumed to be parallel to the X-axis. Hence, the same scan line is exposed by light source pair formed by the light-emitting portions A1 and A2. The scan lines are also identical for the light source pairs formed by B1 and B2, C1 and C2, and D1 and D2. The surface-emitting lasers shown in FIG. 16 are therefore capable of simultaneously exposing four scan lines disposed in a sub-scanning direction (the direction substantially perpendicular to the main scanning direction). Note also that the distance between (centers of) a first light-emitting portion and a second light-emitting portion in each light-source pair is X1.
FIG. 17 is a diagram showing a relationship between scan lines and light-emitting portions in the related art. FIG. 17 shows that, when the first to third scans are performed, twelve scan lines (four scan lines in each of the three scans) are formed. In the first scan, Line 1 to Line 4 are exposed, giving a total of four scan lines. In the second scan, Line 5 to Line 8 are exposed. In the third scan, Line 9 to Line 12 are exposed.
Note that the Lines 1, 5 and 9 are exposed using the light source pair formed by the light-emitting portions D1 and D2. The Lines 2, 6 and 10 are exposed using the light source pair formed by the light-emitting portions C1 and C2. The Lines 3, 7 and 11 are exposed using the light source pair formed by the light-emitting portions B1 and B2. The Lines 4, 8 and 12 are exposed using the light source pair formed by the light-emitting portions A1 and A2.
A distance (pitch) Psub between two adjacent scan lines on the photosensitive body, a distance (row gap) P between the light-emitting portions, and an optical magnification Msub in the sub-scanning direction of the scanning optical system are related by the following equation.Psub=Msub×P 
The row gap P between the light-emitting portions is a value determined by the resolution of the image forming apparatus. For instance, if the resolution is 2400 DPI, the pitch Psub between the scan lines is as follows.
  Psub  =            25.4      /      2400        =    0.0105833  In other words, the pitch Psub is approximately 10.58 μm. The optical magnification Msub in the sub-scanning direction of the scanning optical system, on the other hand, is related to an optical efficiency.
Note that it is hard for a surface-emitting laser to output a larger amount of light than a laser with the edge emitter construction. This is because the semiconductor substrates are thin, and it is not therefore possible to form an oscillator large enough to perform the laser oscillation necessary to produce a larger amount of light. Thus, to compensate for the small amount of light outputted from the surface-emitting laser, optical efficiency and optical magnification have to be increased. For example, when the optical magnification Msub is set to ×3, the row gap P is as follows.
  P  =            Psub      /      Msub        =                  0.0105833        /        3            =      3.53      
Thus, the row gap P is approximately 3.53 μm.
A mask of semiconductor exposure apparatus used when manufacturing the laser devices is generally accurate to ±1 μm. Consequently, the manufacturing error in the light-emitting portions is ±1 μm. In other words, the row gap P can be anywhere from 2.53 μm to 4.53 μm in width. By a similar calculation, the error in the pitch Psub on the photosensitive body is ±3 μm due to the effects of the optical magnification. Errors of the order of 3 μm in the image causes unevenness of pitch, which leads to a deterioration in image quality. The unevenness of pitch is generated periodically with a period that depends on the number of beams and the number of polygon surfaces. Hence, when the number of beams is increased for the multibeam method, the spatial frequency of the unevenness of pitch also increases. Consequently, it is easy to visually confirm the unevenness of pitch. Conceivable methods for improving the manufacturing precision of the light-emitting portions include improving the precision of the semiconductor masks and the use of steppers in the manufacturing process. However, these both have the disadvantage of causing large increases in cost.